Metro apps are always suspended when not in the foreground. I strongly recommend watching the BUILD video on
How and when you app will run if you are planning on making Metro apps, as the model is very different from traditional Win32 apps.

in my case it happened when I installed WDP and was not able to get all my driver updates, specifically my display driver. Please check for driver issues in your Action Center and Device Manager keeping a special focus on display/graphic drivers.